/usr/share/sgml/dtd/xml-core/catalog.dtd is in xml-core 0.13+nmu2.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/sgml/dtd/xml-core/../../../xml/schema/xml-core/catalog.dtd
/usr/share/sgml/dtd/xml-core/../../../xml/schema/xml-core/catalog.dtd